1. Field of the Invention
This invention relates to a unit for driving an objective lens in an optical disk apparatus by which light beams such as laser beams always converge on a recording medium through the objective lens, thereby attaining optical recording, optical playing, and optical erasing of information on the recording medium.
2. Description of the Prior Art
FIG. 17 shows a conventional magneto-optical disk apparatus in which an optical disk 600 is turned by a motor 900, and a laser beam 400 from a laser light source 300 is reflected by a mirror 500 and then converges on the surface of a recording medium 606 included in the magneto-optical disk 600 via an objective lens 200 thereby attaining playing, erasing and writing of information thereon. The objective lens 200 is moved in the direction vertical to the surface of the recording medium 606 of the magneto-optical disk 600 (i.e., the focusing direction) or in the radical direction of the magneto-optical disk 600 (i.e., the tracking direction) by a unit 100 for driving the objective lens in such a manner that the laser beam 400 passing through the objective lens 200 can always converge on the recording track of the recording medium 606. The above-mentioned optical system is contained within an optical head 700. As shown in FIG. 18, the objective lens-driving unit 100 is provided with a supporting means 110 for supporting the objective lens 200, e.g., a cylindrical holder, which is movable in the focusing direction or the direction of arrow y (i.e., in the direction of the optical axis of the objective lens 200) with the below-mentioned intermediate supporting means 130. The cylindrical holder 110 is fixed to the intermediate supporting means 130 by a pair of parallel springs 120 that are movable in the tracking direction or the direction of arrow x (i.e., in the direction of the radius of the disk 600). An elastic substance 140 is disposed inside of each of the springs 120. A counterbalance 160 is positioned outside of the lower portion of the cylindrical holder 110.
A pair of magnetic circuits 160 are fixed to the housing 170 of the objective lens-driving unit 100 in such a manner that they are symmetrical with regard to the optical axis of the objective lens 200. Each of the magnetic circuits 160 is composed of a yoke 180 and a magnet 190 and a yoke plate 166. The yoke plate 166 and the yoke 180 form a gap 220 therebetween. A coil 210, one end of which is fixed to the cylindrical holder 110, is positioned within the gap 220.
Thus, when current applied to the above-mentioned coil 210 is changed, an electromagnetic force is created to move the coil 210 such that displacement of both the cylindrical holder 110 and the objective lens 200 supported by the cylindrical holder 110 in the tracking direction can be achieved.
On the other hand, a driving means 299 for driving the objective lens 200 in the focusing direction is placed downward of the above-mentioned intermediate supporting means 130. The driving means 299 is composed of a magnetic circuit 250 composed of a magnet 270, yoke plate 260, yoke 280, and a coil 300, one end of which is fixed to the lower end of the intermediate supporting means 130 and which is positioned within a gap 290 formed between the yoke plate 260 and the yoke 280. The intermediate supporting means 130 is supported by a pair of springs 230 such as circular leaf springs in a manner to be movable in the focusing direction. An elastic substance 240 is disposed on the springs 230.
In the conventional objective lens-driving unit 100 with the above-mentioned structure, if the stiffness in the focusing direction of each spring 120 that is movable in the tracking direction and the stiffness in the tracking direction of each spring 230 that is movable in the focusing direction are insufficient, and/or if the center G of gravity of the movable portion 111 for driving the objective lens 200 in the tracking direction comprising the objective lens 200, the objective lens-supporting means 110 and the coil 210 is displaced from the center C of the movable portion 111 at which the driving force in the tracking direction acts, the movable portion 111 will turn around its center G of gravity, causing turning of a movable portion 222 for driving the movable portion 111 in the focusing direction, which comprises the intermediate supporting means 130 and the coil 300, via the parallel springs 120, so that rotation resonance arises in the springs 120, which causes distortion in the frequency characteristics of the magneto-optical disk apparatus. This becomes trouble-some in the servomechanism of the said magneto-optical disk apparatus. In order to remove such a problem, the center G of gravity of the movable portion 111 must be adjusted by use of a counterbalance 150 so as to match the center C of the movable portion 111 at which the driving force in the tracking direction acts.
However, when a rectangular magnetic circit is used as the magnetic circuit 160 as shown in FIG. 19, the objective lens-supporting means 110 vibrates, due to the deflection of the parallel springs 230, in the focusing direction so that the gap 183 formed between the yoke 180 and the yoke plate 181 must be made sufficiently large so as to permit the coil to move freely within the gap 183, causing lack of uniformity in the magnetic flux density therein. Thus, depending upon the positions of the upper portion 211 and the lower portion 212 of the coil 210 in the focusing direction within the gap 183, a difference arises between the driving force produced to move the upper portion 211 of the coil 210 in the tracking direction and the driving force produced to move the lower portion 212 of the coil 210 in the tracking direction, which causes a displacement of the center of the movable portion 111 at which the driving force in the tracking direction acts.
Moreover, since the width d.sub.1 of the coil 210 is greater than that of the fixed magnetic field represented by the width d.sub.2 of the yoke plate 181, the center of movable portion 111, at which the driving force that is created in the tracking direction acts, depends upon the position of the magnetic circuit. However, the center of gravity of the movable portion 111 that is movable in the tracking direction moves up and down with a displacement of the movable portion 222 that is movable in the focus direction, resulting in a displacement of the center of gravity of the movable portion 111 from the center of the movable portion 111 at which the driving force created in the tracking direction acts, so that a turning rotation resonance will arise in the springs 120. In order to prevent such a resonance, even if the movable portion 111 is balanced by a counterbalance 150, the movement of the movable portion 111 in the tracking direction when the movable portion 222 in the focusing direction is lifted is opposite to that of the movable portion 111 in the tracking direction when the movable portion 222 is lowered, so that such a turning resonance will unavoidably occur.
A coil that attains a feedback control of the velocity of the coils 210 can be used in the objective lens-driving unit 100. For this purpose, one of the two coils 210 is used for driving the movable portion 111 in the tracking direction and the other is used for the feedback control of the velocity thereof such that the output power for the feedback control coil is returned to the said tracking-direction driving coil via an appropriate circuit, which causes an enlargement of the size of the unit 100.
FIG. 21 is a partly cross-sectional view of the conventional objective lens-driving unit 100 shown in FIG. 18, in which four lead wires 901 which are a part of the tracking-direction driving coils 210 to supply electrical power to the tracking-direction driving coils 210, are led to the outside of the unit 100 through the hole 171 of the housing 170. Parts of the focusing-direction driving coils 300 are also used as lead wires 902 through which electrical power is supplied to the focusing-direction driving coils 300, and each lead wire 902 is led to the outside of the unit 100 through the hole 171 of the housing 170. Since the wires are exposed to the outside, there is a possibility that they will be damaged, causing difficulties in maintenance.
On the other hand, a velocity-detecting apparatus using an electro-magnetic coil therein has been proposed, in which, if a coil for driving the said coil in a certain direction is also used as a coil for detecting the velocity of the said driven coil, the velocity-detecting coil cannot be used as a driving coil (or a driven coil) during the detection of velocity, and accordingly current for driving the driving coil and current for detecting the velocity of the driven coil must be separately supplied to these coils, which causes complexity in the structure of the velocity-detecting apparatus and increases the production cost of this apparatus.
Therefore, when this velocity-detecting apparatus is incorporated into the above-mentioned conventional objective lens-driving unit, the unit becomes complex and expensive.